1. Field of the Invention
The present invention relates to a crystal growing apparatus for growing a Group III nitride-based compound semiconductor crystal through the flux method.
2. Background Art
Conventionally known crystal growing apparatus and methods for growing Group III nitride-based compound semiconductor crystals through the flux method are disclosed in, for example, Japanese Patent Application Laid-Open (kokai) Nos. 2001-58900, 2001-64097, 2001-64098, 2001-102316, 2002-201100, 2004-168650, and 2006-041458.
Generally, when a Group III nitride-based compound semiconductor crystal is grown through the flux method, a crucible which contains a flux material is placed in an electric furnace, which serves as a crystal growing chamber of high temperature and high pressure. Then, a crystal growing process is started. In this connection, according to a most common, rational practice, a glove box is connected to an inlet of the electric furnace, and the crystal growth atmosphere in the electric furnace and the atmosphere in the glove box are integrally controlled so that the atmospheres have almost the same mixing ratio of gases at least when the furnace is opened.
According to a most commonly practiced procedure of working on a flux material, solid masses of sodium (Na) which have been highly purified are placed in the aforementioned glove box; after the internal atmosphere of the glove box is adjusted to predetermined conditions, oxidized surface layers of the solid masses of sodium (Na) are scraped off; and the thus-treated solid masses of sodium (Na) are placed in the aforementioned crucible, followed by start of crystal growth through the flux method. Thus, a knife must be used in the glove box for scraping off oxidized surface layers from the solid masses of sodium (Na).
In order to produce a semiconductor crystal having sufficiently good quality through the flux method, the purity of sodium (Na) used as a flux material is desirably at least 4N (99.99%), more desirably 6N or higher. Japanese Patent Application Laid-Open (kokai) No. 2006-041458 discloses an oxide-removing method different from the aforementioned scraping-off method. Specifically, solid masses of sodium (Na) are melted within the glove box through application of heat. Oxide and the like appearing on the surface of molten sodium (Na) are removed for purifying sodium (Na).
However, in the process of scraping off oxidized surface layers from the solid masses of sodium (Na), a small part of a knife is scraped, and the scrapings are mixed in the flux material, potentially resulting in a great drop in purity of flux. Also, this method increases in material cost with respect to sodium (Na), since a not small portion of a solid mass of sodium (Na) must be scraped off.
Furthermore, when, within the glove box, solid masses of sodium (Na) are formed into anglewise shapes as a result of scraping-off of oxide surface layers and are then placed in a crucible, the contained solid masses of sodium (Na) form a lot of interspace. This tends to lower the bulk ratio of the flux material to the volume of the crucible. Since improvement of the bulk ratio (i.e., reduction of the interspace within the crucible to a maximum extent to a maximum extent) requires a lot of time and effort, work efficiency is very hard to improve.
Meanwhile, a process of melting and purifying sodium (Na) within the glove box as described in, for example, Japanese Patent Application Laid-Open (kokai) No. 2006-041458 involves the following problems.
Problem 1: In the process of spooning up impurities appearing on the surface of molten liquid sodium (Na) by use of a spoon or the like, some portion of liquid sodium (Na) is unavoidably spooned up together with the impurities, thereby wasting the flux material.
Problem 2: Spooning up impurities stirs liquid sodium (Na) with a spoon or the like; consequently, some portion of impurities, such as oxide, is brought deep into liquid sodium (Na). Therefore, complete removal of impurities is difficult. Even in an attempt to remove only a surface layer of impurities by use of a suction device or the like, the same problem arises due to convection.